Dental implant systems generally include three major components: an implant, a coping or abutment, and a cast on structure (e.g., a crown).
The implant itself is generally made of Ti and generally has both external and internal threads. The implant is screwed into a hole that has been drilled into the jaw. Through a process called osseointegration, the TiO2 that naturally forms on the outer surface of the external implant threads chemically bonds to the bone. This process can be enhanced via a number of chemical coatings.
On top of the implant is an abutment or coping. This is a precision-machined component that serves a number of important functions. First, it generally has a number of geometric features such as a hex, square, etc, that mate with a similar feature on the implant. This serves to properly orient the abutment when it is placed on the implant and to maintain that geometric relationship throughout the fabrication and installation process. Second, the abutment serves as a base for holding additional material that forms the tooth anatomy or crown. Third, the abutment is attached to the implant using a screw that attaches to internal threads within the implant. The screw technique is favored because it allows for potential replacement of the abutment/tooth structure without the need to physically remove the implant from the jaw.
The abutment also serves as the carrier for the cast on structure created by the dentist or dental lab to mimic the natural anatomy of a tooth. Generally, the dentist will take an impression of the patient's mouth and create a wax model of the tooth geometry that they wish to create for the tooth. The wax model is formed on top of the abutment. Wax sprues are attached to tooth model and the assembly is invested into a refractory slurry and allowed to dry. The sprues are designed to exit one end of the investment once it has fully hardened. This unit is placed into a burnout oven and the wax is evaporated from the unit, thereby creating a negative three-dimensional image of the tooth anatomy and sprues. The sprues create a path for casting molten metal onto the abutment. Depending on the type of alloy used, casting temperatures can range from below 1000° C. to over 1400° C. (1800° F. to 2550° F.).
Because the abutment must maintain the precise seating geometry to minimize any crevices from forming between the abutment and the implant, it is important that the abutment does not distort or soften significantly during the cast on process. Otherwise, any such pockets could provide sites for bacterial growth. The seating surface also acts to transfer chewing stresses from the crown to the jaw. Asymmetric stresses associated with warping of the seating surface can reverse the osseointegration process. A high solidus temperature tends to help reduce thermal distortion during casting.
After casting, crown and bridge (“C&B”) alloys may be polished and placed in the mouth with the natural metal finish exposed. However, in many applications, the patient prefers the look of a natural tooth. In these cases, the tooth anatomy and aesthetics are developed by placing multiple layers of porcelain over top of the casting. This practice is called porcelain fused to metal (“PFM”) or PFM restorations. The porcelain firing process uses multiple high temperature cycles in the range of 980° C. (1800° F.). Because of the need to maintain shape during the porcelain firing, PFM alloys tend to have higher solidus temperatures than the C&B alloys, and therefore are cast on to the abutment using higher casting temperatures. The porcelain firing is also done in a temperature range that can anneal and soften the abutment, thereby reducing its ability to stand up to the high chewing stresses without mechanical distortion.